Method and system for physiologic control of an implantable blood pump

ABSTRACT

A physiologic control method and system for an implantable blood pump includes operating the pump at a predetermined speed and monitoring the patient&#39;s diastolic pump flow rate. The predetermined speed is varied in response to the diastolic pump flow rate. The pump speed may further be adjusted in response to the patient&#39;s heart rate. The speed may be increased and decreased in response to corresponding changes in the diastolic pump flow rate, and increased in response to an increase in heart rate.

This application claims the benefit of U.S. Provisional Application No.60/346,721, filed on Jan. 7, 2002, the entire contents of which isincorporated by reference.

BACKGROUND OF THE NVETION 1. FIELD OF THE INVENTION

The invention relates generally to implanted or implantable blood pumpsystems, and more specifically, to a method and system for physiologiccontrol of such pumps. 2. DESCRIPTION OF RELATED ART

Generally, implantable blood pump systems are employed in either of twocircumstances. First an implantable blood pump may completely replace ahuman heart that is not functioning properly, or second, an implantableblood pump may boost blood circulation in patients whose heart is stillfunctioning although pumping at an inadequate rate.

For example, U.S. Pat. No. 6,183,412, which is commonly assigned andincorporated herein by reference in its entirety, discloses a ventricleassist device (VAD) commercially referred to as the “DeBakey VAD®.” TheVAD is a miniaturized continuous axial-flow pump designed to provideadditional blood flow to patients who suffer from heart disease. Thedevice is attached between the apex of the left ventricle and the aorta.

Known implantable blood pump systems typically are controlled in an openloop fashion where a predetermined speed is set and the flow rate variesaccording to the pressure differential across the pump. The pump itselfmay be controlled in a closed loop fashion, wherein the actual pumpspeed is fed back to a motor controller that compares the actual speedto the desired predetermined speed and adjusts the pump accordingly.However, prior art closed loop control systems—varying the pump speed inresponse to a monitored physiologic parameter—have largely beenunsatisfactory.

For example, some systems have attempted to use a patent's heart rate,or pulse, as a physiologic control trigger—the pump speed set point isvaried in response to the patient's heart rate . Other systems attemptto vary the pump speed based on the variation of the VAD pump's flow orcurrent signals with respect to the signal's mean value or with respectto pump speed. For example, a “pulsatility index” is derived

$( {e.g.\mspace{14mu}\frac{{Signal}_{{MA}\; X} - {Signal}_{{MI}\; N}}{{Signal}_{MEAN}}} )$and compared to a predetermined threshold and the pump speed is variedaccordingly.

Unfortunately, these physiologic control methods have not provided anadequate closed loop control parameter, as it appears that knownphysiologic control parameters such as these do not necessarily varyproportionally to a patient's level of activity—i.e., the patient'sdemand for increased blood flow. Further, while a patient's heart ratemay increase during exercise, heart rate may be controlled by otherfactors, such as medication or a pacing device. Still further, thepatient may not have significant native heart rate function, preventingthe heart rate from increasing in response to the body's demand forincreased blood flow. Moreover, there exists some evidence that apatient's heart rate may decrease as the pump's speed is increased.Hence, heart rate alone may not provide a satisfactory physiologiccontrol parameter.

The present invention addresses shortcomings associated with the priorart.

SUMMARY OF THE INVETION

Aspects of the present invention concern a physiologic control systemand method for a blood pump system such as a VAD system. The pump systemincludes, for example, an implantable pump such as a VAD and acontroller for controlling the pump. The system may firther include animplantable flow measurement device. The control method includesoperating the pump at a predetermined speed and monitoring the patient'sdiastolic VAD flow rate. In exemplary embodiments, the peak diastolicVAD flow rate, average diastolic VAD flow rate, and/or the average peakdiastolic VAD flow rate is monitored. The predetermined speed is variedin response to the diastolic VAD flow rate. The pump speed may furtherbe adjusted in response to the patient's heart rate. In certainembodiments, the speed is increased and decreased in response tocorresponding changes in the diastolic VAD flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 schematically illustrates various components of an implantablepump system in accordance with embodiments of the present invention;

FIG. 2 is a cross-section view of an exemplary implantable pump inaccordance with embodiments of the present invention;

FIG. 3 is a block diagram illustrating aspects of a controller module inaccordance with embodiments of the present invention; and

FIG. 4 is a chart illustrating time plots of various physiologicparameters, showing the various parameters' responses to the onset andconclusion of exercise;

FIG. 5 is a block diagram schematically illustrating a flow processingsystem in accordance with embodiments of the invention; and

FIG. 6 is a flow diagram illustrating a physiologic control method inaccordance with embodiments of the invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVETION

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Turning to the figures, FIG. 1 illustrates a ventricular assist device(VAD) system 10 such as disclosed in U.S. Pat. No. 6,183,412, which iscommonly assigned and incorporated herein by reference in its entirety.The VAD system 10 includes components designed for implantation within ahuman body and components external to the body. Implantable componentsinclude a rotary pump 12 and a flow sensor 14. The external componentsinclude a portable controller module 16, a clinical data acquisitionsystem (CDAS) 18, and a patient home support system (PHSS) 20. Theimplanted components are connected to the controller module 16 via apercutaneous cable 22.

The VAD System 10 may incorporate an implantable continuous-flow bloodpump, such as the various embodiments of axial flow pumps disclosed inU.S. Pat. No. 5,527,159 or in U.S. Pat. No.5,947,892, both of which areincorporated herein by reference in their entirety. An example of ablood pump suitable for use in an embodiment of the invention isillustrated in FIG. 2. The exemplary pump 12 includes a pump housing 32,a diffuser 34, a flow straightener 36, and a brushless DC motor 38,which includes a stator 40 and a rotor 42. The housing 32 includes aflow tube 44 having a blood flow path 46 there through, a blood inlet48, and a blood outlet 50.

The stator 40 is attached to the pump housing 32, is preferably locatedoutside the flow tube 44, and has a stator field winding 52 forproducing a stator magnetic field. In one embodiment, the stator 40includes three stator windings and may be three phase “Y” or “Delta”wound. The rotor 42 is located within the flow tube 44 for rotation inresponse to the stator magnetic field, and includes an inducer 58 and animpeller 60. Excitation current is applied to the stator windings 52 togenerate a rotating magnetic field. A plurality of magnets 62 arecoupled to the rotor 42. The magnets 62, and thus the rotor 42, followthe rotating magnetic field to produce rotary motion.

FIG. 3 conceptually illustrates aspects of the pump system 10. Morespecifically, portions of the controller module 16 and the pump 12 areshown. The controller module 16 includes a processor, such as amicrocontroller 80, which in one embodiment of the invention is a modelPIC16C77 microcontroller manufactured by Microchip Technology. Themicrocontroller 80 includes a multiple channel analog to digital (A/D)converter, which receives indications of motor parameters from the motorcontroller 84. Thus, the controller module 16 may monitor parameterssuch as motor current, the VAD flow rate, and motor speed.

The embodiment shown in FIG. 3 further includes an integral flow meter124. At least one flow sensor 14 is implanted down stream of the pump12. Alternately, a flow sensor 14 may be integrated with the pump 12.The flow meter 124 is coupled between the implanted flow sensor 14 andthe microcontroller 80. The flow meter 124 receives data from the flowsensor 14 and outputs flow rate data to the microcontroller 80, allowingthe system to monitor instantaneous flow rate.

Since the implanted flow sensor 14 is coupled to the flow meter 124 ofthe controller module 16, a true measure of system performance (flowrate) is available for analysis, in addition to pump parameters such asmotor speed and current (power). Further, since the flow meter 124 is anintegral component of the controller module 16, VAD flow rate may bedisplayed on the controller module display and VAD flow rate data may besaved in the controller module memory.

In exemplary embodiments of the invention, the motor controller 84comprises a MicroLinear ML425 Motor Controller provided by FairchildSemiconductor. The operation of the brushless DC motor 38 of the presentinvention requires that current be applied in a proper sequence to thestator windings 52 to create the rotating field. Two stator windings 52have current applied to them at any one time, and by sequencing thecurrent on and off to the respective stator windings 52, the rotatingmagnetic field is produced. In an embodiment of the invention, the motorcontroller 84 senses back electromotive force (EMF) voltage from themotor windings 52 to determine the proper commutation phase sequenceusing phase lock loop (PLL) techniques. Whenever a conductor, such as astator winding 52, is “cut” by moving magnetic lines of force, such asare generated by the magnets 62 integrated into the rotor of thebrushless DC motor 38, a voltage is induced. The voltage will increasewith rotor speed 42. It is possible to sense this voltage to determinethe rotor 42 position in one of the three stator windings 52 becauseonly two of the motor's windings 52 are activated at any one time.

An alternative method of detecting the rotor 42 position relative to thestator 40 for providing the proper stator winding 52 excitation currentsequence is to use a position sensor, such as a Hall Effect sensor.Implementing aspects of the present invention using a motor with rotorposition sensors, rather than a sensorless motor, would be a routineundertaking for one skilled in the art having the benefit of thisdisclosure. However, adding additional components, such as Hall Effectsensors, requires additional space, which is limited in any implanteddevice application. Further, using a position detection device addssources of system failures.

The actual pump speed is determined and fed back to the controllermodule 16, which compares the actual speed to a desired predeterminedspeed and adjusts the pump 12 accordingly. In accordance with certainembodiments of the invention, the pump 12 is controlled in a closed loopfashion wherein the desired pump speed is varied for events such assleeping, normal activity or high energy exertion.

The contraction phase of the heart beat is referred to as systole, therelaxation phase is referred to as diastole. Thus, the systolic VAD flowis the maximum VAD flow rate, while the diastolic VAD flow rate is theminimum VAD flow rate. It has been determined (empirically) that apatient's diastolic VAD flow rate significantly increases at the onsetof exercise, and decreases at the conclusion of exercise. In comparison,the systolic VAD flow rate, for example, remains relatively constant atthe onset and conclusion of exercise. Thus, in certain embodiments ofthe invention, the pump speed is adjusted in response to changes in thediastolic VAD flow rate.

The contraction phase or pumping phase of the cardiac cycle is referredto as systole, the relaxation phase or filling phase is referred to asdiastole. In healthy, non-VAD patients, there is positive blood flow,from the left ventricle through the aortic valve, during systole and noblood flow, from the left ventricle through the aortic valve, duringdiastole. However, in patients who have been implanted with a left VADthere is generally positive flow through the VAD during both systole anddiastole. This is because the implanted continuous flow VAD essentiallyadds a constant positive flow offset to the native heart's pulsatileflow contribution.

Therefore, the conventional definitions for systolic flow and diastolicflow must be modified to make them applicable to patients implanted withleft VADs. Thus, the systolic flow rate is considered herein as the flowcontribution above the mean flow rate value, while the diastolic VADflow rate is considered herein as the VAD flow contribution below themean VAD flow rate. Peak systolic VAD flow rate is considered herein tobe the maximum VAD flow rate value in the VAD flow rate waveform in onecardiac cycle and average peak systolic VAD flow rate is the averagevalue of multiple peak systolic VAD flow rate values over severalcardiac cycles. Similarly, peak diastolic VAD flow rate is consideredherein to be the minimum VAD flow rate value in the VAD flow ratewaveform in one cardiac cycle and average peak diastolic VAD flow rateis the average value of multiple peak diastolic VAD flow rate valuesover several cardiac cycles.

It has been determined that a patient's peak diastolic VAD flow rate oraverage peak diastolic VAD flow rate significantly increases at theonset of exercise, and decreases at the conclusion of exercise. Thus, incertain embodiments of the invention, the pump speed is adjusted inresponse to changes in the peak diastolic VAD flow rate or average peakdiastolic VAD flow rate.

FIG. 4 provides time plots of various physiologic parameters, includingheart rate 201, peak systolic VAD flow rate 202, mean VAD flow rate 203,peak diastolic VAD flow rate 204, average peak to peak VAD flow (VADflow maximum-VAD flow minimum) 205, and average pulsatility index 206.Each plot includes rest 210, exercise onset 212, and exercise conclusion214 points for the patient. As shown in FIG. 4, the peak diastolic VADflow plot 204 shows the greatest change in response to the onset andconclusion of exercise.

Thus, in accordance with embodiments of the invention, the patient'sdiastolic VAD flow rate is monitored and the controller module 16 isprogrammed to increase the speed of the pump 12 in response to anincrease in diastolic VAD flow, and decrease the pump speed in responseto a decrease in diastolic VAD flow. In specific embodiments, thepatient's peak diastolic VAD flow rate or average peak diastolic VADflow rate is monitored and the pump speed is controlled in responsethereto.

FIG. 5 illustrates an analog flow processing system 250 in accordancewith an exemplary embodiment of the invention. The system 250 accepts ananalog voltage input signal 252 that is proportional to blood VAD flowrate and generates a digital output signal 254 to indicate when apatient has begun/finished exercising.

The VAD flow signal 252 is ac coupled to a precision rectifier 256 toremove the mean VAD flow rate component from the analog VAD flow signal252. The systolic VAD flow rate 260 and diastolic VAD flow rate 261 areextracted separately. The isolated systolic and diastolic VAD flowsignals 260,261 are then low-pass filtered 262 to yield respectiveaverage peak values of the systolic and diastolic VAD flow rates. Asnoted herein, a patient's peak diastolic VAD flow rate or average peakdiastolic VAD flow rate increases during exercise and decreases at rest.Thus, peak diastolic VAD flow rate or the average peak diastolic VADflow rate is applied to a voltage comparator 264 to compare the signalto a predetermined threshold 266 and provide the binary indication 254of when the patient is exercising. The pump speed may then be adjustedaccordingly.

Although the system 250 illustrated in FIG. 5 is based on processing ananalog signal proportional to VAD flow rate, it would be a routineundertaking for one skilled in the art having the benefit of thisdisclosure to apply the same or a similar technique digitally to processVAD flow rate information as discrete sampled data.

As noted herein, heart rate by itself is not believed to be an exclusivephysiologic indicator for changing the speed of the pump. However, inexemplary embodiments of the invention, heart rate in combination withdiastolic VAD flow rate is used as a physiologic indicator. Thisprovides improved control in patients whose heart rates varyproportionally to their degree of physical activity, while stillallowing physiologic control in patients whose heart rate is controlledby medication or by stimulation from a cardiac pacemaker.

In certain embodiments, an increase in the diastolic VAD flow rate or anincrease in the heart rate may be used to trigger an increase in pumpspeed due to an increase in physical activity. However, only a decreasein diastolic VAD flow is used as the indication of a decrease inphysical activity resulting in decreasing the speed of the pump 12. Thepump 12 is controlled in this manner since it is unknown whether asubsequent decrease in heart rate is the result of a decrease inphysical activity or because the speed of the pump 12 had beenpreviously increased. This is because increases in pump speed typicallyresult in a corresponding increase in mean VAD flow rate and thus anincrease in the perfusion of oxygen to the body. The patient's nativeheart rate may therefore decrease (naturally) when the VAD's flowcontribution is elevated.

FIG. 6 is a flow diagram illustrating a physiologic control method inaccordance with embodiments of the invention. The illustrated methodmonitors both diastolic VAD flow 301 (for example, peak diastolic VADflow rate or average peak diastolic VAD flow rate) and heart rate 302.In block 310, the VAD flow rate is acquired, typically by receiving theVAD flow signal from the flow meter 124. In the particular embodimentillustrated, the flow signal comprises an analog voltage signal that isproportional to the VAD blood flow rate, though other implementationsare envisioned in which a digital signal is received. The DC componentof the signal is removed in block 312, and the diastolic VAD flow rateis extracted from the flow signal in block 314. This information may beprocessed in the manner described and illustrated in FIG. 5.

In block 316, the peak diastolic VAD flow rate or average peak diastolicVAD flow rate is computed, and this value is applied to the baselinevalue in block 318. If the diastolic VAD flow rate has not significantlychanged as compared to the baseline (determined in decision block 320),the system continues to monitor the flow information. If the diastolicVAD flow rate has decreased (decision block 322), the pump speed isdecreased in block 324 and a new baseline established. If the VAD flowrate has increased, the pump speed is increased in block 326 and a newbaseline established.

As noted above, the patient's heart rate may also be monitored, and thisinformation may also be used for physiologic control of the pump. Inblock 340, the heart rate information is acquired, and the average heartrate (instantaneous heart rate is also applicable) is computed in block342. The average is applied to the baseline average (block 344). Theaverage heart rate computed in block 342 is compared to the baseline indecision block 346, and if the rate has not increased, the systemcontinues to monitor the heart rate.

If the average heart rate has increased, the pump speed is increased inblock 326 and a new baseline established. The outputs of decision blocks322 and 346 are applied to an OR gate 348, so that if either the heartrate or diastolic VAD flow rate has increased, the pump speed isincreased in block 326. However, the pump speed is decreased only inresponse to a negative change in the diastolic VAD flow rate (block324).

As noted herein above with reference to FIG. 3, the implantable pumpsystem 10 may include an implantable flow measurement device 14. Inembodiments including the implantable flow sensor 14, the flow rateinformation, and thus the diastolic VAD flow rate information, may beobtained from data the provided by the flow sensor 14 (and flow meter124 where applicable).

However, the diastolic VAD flow rate information may be obtained throughseveral methods. For example, some embodiments include an implantablepressure sensor, and the pressure data may be used to derive flow rateinformation. Still further, in other embodiments, other pump signals aremonitored and analyzed to extract flow rate information.

The controller 16 receives and monitors various system parameters, suchas the pump motor voltage and current (power), the pump speed, flowrate, etc. These signals are time-continuous band-limited signals. Thecurrent signal is a composite signal containing the patient's heart rate(assuming the heart is beating) and other frequencies relating tocertain physiologic responses within the patient's cardiovascular systemsuch as valve openings and closures, changes in systemic resistance,etc.

The power signal is the product of the pump motor current and pump motorvoltage (a constant scalar) and is therefore a composite signal thatcontains information similar to the current signal. The speed signaltypically contains the heart rate of the patient (assuming the heart isbeating) as the dominant frequency along with other frequencies relatedto certain physiologic similar to those discussed above. The flow signalalso typically contains the heart rate of the patient and otherfrequencies related to physiologic responses within the patient'scardiovascular system. Thus, the patient's heart rate information may beextracted from any of several signals available to the controller module16 for use in a physiologic control scheme.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. A method of controlling a blood pump implanted in a patient,comprising: operating the pump at a predetermined speed; monitoring thepatient's pump flow rate; extracting the patient's diastolic pump flowrate from the pump flow rate, wherein the diastolic pump flow rate is aseparately isolated flow contribution below a mean pump flow rate; andchanging the predetermined speed in response to the diastolic pump flowrate, wherein changing the predetermined speed includes increasing thepump speed in response to an increase in the diastolic pump flow rate.2. The method of claim 1, further comprising: monitoring the patient'sheart rate; and changing the predetermined speed in response to theheart rate.
 3. The method of claim 2, wherein changing the predeterminedspeed includes increasing the pump speed in response to an increase inthe heart rate.
 4. The method of claim 2, wherein changing thepredetermined speed includes increasing the pump speed in response to anincrease in the diastolic pump flow rate.
 5. The method of claim 2,wherein changing the predetermined speed includes decreasing the pumpspeed in response to a decreasing in the heart rate.
 6. The method ofclaim 2, wherein changing the predetermined speed includes decreasingthe pump speed in response to a decrease in the diastolic pump flowrate.
 7. The method of claim 1, wherein changing the predetermined speedincludes decreasing the pump speed in response to a decrease in thediastolic pump flow rate.
 8. The method of claim 1, further comprising:setting the predetermined speed of the pump in accordance withactivities performed by the patient.
 9. The method of claim 8, whereinthe activities are sleeping, normal activity or high energy exertion.10. A pump system, comprising: a pump; and a controller having an inputfor receiving a blood pump flow rate signal, the controller beingprogrammed to extract a separate diastolic pump flow rate from the bloodpump flow rate signal and provide a control signal to the pump to varythe speed of the pump in response to the separate diastolic pump flowrate, wherein the separate diastolic pump flow rate is a flowcontribution below a mean flow rate.
 11. The pump system of claim 10,further comprising an implantable flow measurement device having anoutput for providing the flow rate signal.
 12. The pump system of claim10, wherein the controller is further programmed to vary the speed ofthe pump in response to heart rate changes.
 13. The pump system of claim12, wherein the controller is programmed to increase the speed of thepump in response to an increase in at least one of the separatediastolic pump flow rate or the heart rate.
 14. The pump system of claim13, wherein the controller is programmed to decrease the speed of thepump in response to a decrease in the separate diastolic pump flow rate.15. The pump system of claim 10, wherein the controller is programmed toincrease the speed of the pump in response to an increase in theseparate diastolic pump flow rate.
 16. The pump system of claim 10,wherein the controller is programmed to decrease the speed of the pumpin response to a decrease in the separate diastolic pump flow rate. 17.The pump system of claim 10, further comprising an implantable pressuresensor for providing pressure sensor data to the controller.
 18. Thepump system of claim 17, wherein the pressure sensor data from thepressure sensor is used to derive separate diastolic pump flow rateinformation.
 19. A method of controlling a blood pump implanted in apatient, comprising: monitoring the patient's blood pump flow rate;extracting the patient's diastolic pump flow rate from the pump flowrate, wherein the diastolic pump flow rate is a separately isolated flowcontribution below a mean flow rate; changing a speed of the pump inresponse to the extracted diastolic pump flow rate; and increasing thespeed of the pump in response to an increase in the extracted diastolicpump flow rate.
 20. The method of claim 19, further including the stepof decreasing the speed of the pump in response to a decrease in theextracted diastolic pump flow rate.